34 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



physicochemical world, have tried to adduce a definite causomechani- 

 cal explanation of orthogenesis. The best and most comprehensive 

 types of this explanation are those essentially Lamarckian in principle, 

 in which the direct influence of environmental conditions, the direct 

 reactions of the life stuff to stimuli and influences from the world 

 outside, are the causal factors in such an explanation. But while 

 every naturalist will grant that such factors do change and control 

 in a considerable degree the life of the individual, most see no mechan- 

 ism or means of extending this control directly to the species." 



The above-quoted paragraphs from Jordan and Kellogg^ will 

 serve to place before the reader the general ideas involved in the 

 orthogenesis conception. A brief account of the various special 

 theories of orthogenesis follows: 



Carl von NdgeWs ideas of orthogenesis involve a belief in a sort of 

 mystical principle of progressive development, a something, quite 

 intangible, that exists in organic nature, which causes each organism, 

 to strive for or at least make for specialization or perfect adaptation. 

 This idea of an inner driving and directing force reminds one of the 

 "entelechy" of Driesch, or Bergson's '^ creative evolution." Nageli 

 believed that animals and plants would have developed essentially 

 as they have without any struggle for existence or natural selection. 

 This form of orthogenesis theory, then, is alternative to natural 

 selection. 



Theodore Ewicr''s theory of orthogenesis is more scientific and less 

 mystical than Nageli's. He believed that lines of evolution were not 

 miscellaneous and haphazard, but were confined to a few definite 

 directions, determined at their initial stages not by natural selection 

 but by the laws of organic growth, aided by the inheritance of acquired 

 characters. A new character makes a beginning as would the first 

 step in a slow chemical change, or series of such changes, and it must 

 go through to a fixed end, under given conditions, just as surely as does 

 the chemical process. Only when a given character or line of evolu- 

 tion results in the production of a ver>' positive advantage or dis- 

 advantage to the species does natural selection step in to interfere 

 with orthogenesis. The causes of orthogenesis are said "to lie in the 

 effects of external influences, climate, nutrition, or the given constitu- 

 tion of the organism." 



Actual species-forming, or the breaking-up into specific units of- 

 the orthogenetic lines of change, depends, according to Eimer, upon 



^ Jordan and Kellogg, Evolution and Animal Life (D. Appleton and Company). 



