WHAT EVOLUTION IS 83 



skin are under nervous control, with 

 the result, that the animals can mo- 

 mentarily change colors and patterns 

 and thus, so to speak, exercise this 

 system as muscles may be exercised. 

 In the insect the condition is fixed 

 once for all and the individual is in- 

 capable of modifying it. Fixed con- 

 ditions of this kind are beyond the 

 reach of the Lamarckian principles 

 and form a body of material the evo- 

 lutionary explanation of which must 

 be sought for in other directions. 

 Thus, granting the validity of La- 

 marck's hypothesis, it, nevertheless, 

 falls short of an explanation of all 

 the evolutionary aspects of organic 

 nature and must be supplemented by 

 other factors to reach completion. 



But not only does Lamarckism fail 

 to apply to all classes of instances 

 under organic evolution, it also in- 

 volves, as one of its essentials, the 



