On the Mechanical Conception oj Nature. 687 



natural selection; I mention this only in order to 

 point out that in these cases transformation 

 depends upon a double action of the environment, 

 since the latter first induces small deviations in 

 the organism by direct action, and then accumu- 

 lates by selection the variations thus produced. 



By regarding variability in this manner by 

 considering each variation as the reaction of the 

 organism to an external action, as a diversion of 

 the inherited developmental direction, it follows 

 that without a change in the environment no ad- 

 vance in the development of organic forms can 

 take place. If we imagine that from any period 

 in the earth's history the conditions of life remain 

 completely unchanged, the species present on the 

 earth at this period would not, according to our 

 view, undergo any further modification. Herein 

 is clearly expressed the difference of this view 

 from that other one according to which the in- 

 citing principle of modification is not in the envi- 

 ronment, but lies in the organism itself in the form 

 of a phyletic vital force. 



I cannot here refrain from once more returning 

 to the old (ontogenetic) vital force of the natural 

 philosophers, since the parallel between this and 

 its younger sister, the " phyletic vital force " 

 which appears in so many disguises, is indeed 

 striking. Were the inciting principle of the 

 development of the individual actually an inde- 

 pendent vital force acting within the organism, 



