OTHER THEORIES OF SPECIES-FORMING. 277 



parts theory, and Weismann's theory of germinal selection 



are also in a way theories of orthogenesis : they explain how 



variations begin and continue along fixed lines, but they 



both soon surrender control of descent to natural selection. 



There are, however, two or three theories of 



Different types orthogenesis which have been developed by 



of theories of , , . , , -, i , , 



orthogenesis, their proposers to the degree where they are 

 boldly offered as substitutes for natural selec- 

 tion. Two especially notable theories of this character are 

 those proposed and defended respectively by Nageli and by 

 Eimer. These two are not only the most notable and most 

 completely elaborated of orthogenetic theories but they 

 represent two radically different points of view among the 

 orthogenesists themselves, in that Nageli found his ortho- 

 genesis-producing factor or cause in a somewhat mystical 

 vitalistic inner force, or so-called Vcrvollkoinnungsprinzip, 

 in the__ orgajaism , while Eirner^ finds orthogenesis produced 

 and controlled by the directly working external factors of 

 climate, food supply, and environment generally. Similar 

 conceptions or beliefs regarding the direct and accumulating 

 effect of environmental factors have been presented by Dar- 

 win, Haeckel, Cope, Henslow, Emery, Piepers, Lloyd 

 Morgan, and numerous others. In fact probably a majority 

 of biologists entertain a conviction, often not clearly de- 

 fined and generally unaccompanied by any satisfactory con- 

 ception of a mechanism for achieving what they believe 

 to exist, of the actuality of an influence on organic modifi- 

 cation and descent directly exerted by those various external 

 factors or conditions of organic life which we call, collec- 

 tively, environment. 



Nageli's 10 theory of orthogenesis depends upon the as- 

 sumption of his so-called principle of progressive develop- 

 Nageli's theory ment ( Vervollkomnungsprinsip) , a something 

 of orthogenesis, inherent in the organic world which makes 

 each organism in itself a force or factor making towards 



