OTHER THEORIES OF SPECIES-FORMING. 283 



.as a matter of fact Lamarck's species-forming influences 

 were, chiefly at least, the inherited results of actual use or 

 disuse, or of other functional stimulation initiated or exer- 

 cised actively by the organism itself. In the actual varia- 

 tion of organisms Eimer sees none of that "oscillation" or 

 equal variation around a median or modal point character- 

 istic of the Darwinian conception, but sees always a deter- 

 minate variation in a few definite lines. He denies positively 

 any capacity on the part of natural selection to create species, 

 finding it effective in breaking the continuous organic chain, 

 that is, of separating it into species, only when aided by 

 geographical isolation. The actual species-forming, that is, 

 the breaking up into specific units of the orthogenetic lines 

 of change instituted by his dynamic factors, he finds to de- 

 pend on three chief moments, viz., a standing still or cessa- 

 tion of development (Enturicklungsstillstand) ; a sudden de- 

 velopment by leaps, called halmatogenesis (which is almost 

 exactly the fundamental idea in Korschinsky's and de Vries's 

 later heterogenesis theory) ; and third, a hindrance or difri- 

 'Cttlty in reproduction (which is the essential factor in Ro- 

 manes's theory of physiological selection proposed ten years 

 later). It is of interest to note Eimer 's claim to the original 

 conception of species-forming both by heterogenesis and 

 through physiological selection, with which two theories 

 the names of de Vries and Romanes, respectively, are com- 

 monly associated as those of the original proposers. 



Of Eimer's three species-forming factors he lays most 

 stress on the one I have first mentioned viz., Entwicklungs- 

 stillstand. "The origin of species depends essentially on 

 Entwicklungsstillstand or Genepistase, that is, the standing 

 still of certain forms at definite stages in the developmental 

 line, while others go on." This permits of the origin of 

 numerous different species in the same locality or region, 

 without any need of isolation. As orthogenesis modifies, 

 that is, causes to vary in the same way, many individuals at 



