PhyUtic Parallelism in Mdamorphic Species. 4 1 1 



exists and acts periodically, the variability must 

 always pass over the different stages in a fixed 

 direction, like a wave over the surface of water 

 imago, pupa, and larva, or larva, pupa, and 

 imago, must successively become variable. Cases 

 like that of Arascknia Prorsa, in which all three 

 stages are variable, may certainly be thus ex- 

 plained, but those instances in which the larva 

 and imago are extremely variable, and the pupa 

 quite constant, are entirely inexplicable from this 

 point of view. 



The latter can, however, be very simply ex- 

 plained if we suppose the changes to be depen- 

 dent upon external influences. From this stand- 

 point we not only see how it is possible that an 

 intermediate stage should remain uninfluenced by 

 the changes which affect the two other stages, but 

 we can also understand why it should just be the 

 pupal stage that plays this part so frequently. If 

 we ask why most pupae are constant and are rela- 

 tively but very slightly variable, the answer will 

 be found in the facts that all pupae which remain 

 concealed in the earth or inside plants (Sesiida), 

 or which are protected by stout cocoons, show 

 complete constancy, whilst any considerable 

 amount of variability occurs only in those pupae 

 which are suspended or openly exposed. This is 

 closely connected with a fact to which I have 

 called attention on a former occasion,* viz., that 



"(iber den Einfluss der Isolirung auf die Artbildung." 



, 1872, p. 20. 



