300 THE THEORIES OF EVOLUTION 



but indifferent characters, produced by external con- 

 ditions regardless of their usefulness (physiogenesis) . 

 Such is the colouring of butterflies' wings or molluscs' 

 shells, etc., a character no more useful to butterflies or 

 molluscs, Eimer thinks, than a yellow glimmer is to 

 gold or iridescence is to a soap bubble. Those char- 

 acters which had their origin in almost imperceptible 

 variations are developed by evolution, not, as selec- 

 tionists thought, along various lines, oscillating around 

 a median or modal point, but along one straight line 

 of advance or retrogression. 



The origin of species depends essentially on the 

 standing still of certain forms at definite stages in the 

 developmental line, while others, more sensitive to va- 

 rious external influences that are active at the time, 

 continue the onward march. A break occurs then be- 

 tween forms in the continuous organic chain. This 

 phenomenon which Eimer designates as Genepistase, 

 is considered by him as the main factor in species sepa- 

 ration which is effected without the assistance of geo- 

 graphical isolation. The divergence between species 

 is also increased by the fact that the degree of develop- 

 ment is not uniform in the different organs (Heterop- 

 istase). 



Geographical isolation, physiological isolation 

 (Kyesamechanics), that is, hindrance in reproduction 

 due to some modification of the genitalia, and finally 

 the appearance by saltation of sudden variations due 



