304 Studies in the Theory of Descent. 



difficult to determine, even approximately, the 

 relative proportion of the brown to the green 

 individuals. I have only seen one of the latter 

 in Dr. Staudinger's collection (Compare Fig. 62, 

 PL VII.). 



In Deilephila Vesper tilio, Euphorbia, Dahlii, 

 Mauritanica^ Nic&a, and Galii^ the green form 

 has completely disappeared. The blackish olive- 

 green colour shown by many caterpillars of the 

 two last species, can be considered as a faint 

 retention of the light green colour which they 

 formerly possessed, and which they both show at 

 the present time in their young stages. 



Beginning with the appearance of single darker 

 individuals, we pass on in the first place to a 

 greater variability of colouring, and from this, by 

 the greater diminution of the intermediate forms, 

 to polymorphism ; the complete extermination of 

 these forms ending in dimorphism. The whole 

 process of transformation has been thus effected : 

 As the new colouring always prevailed over the old, 

 the latter was at length completely displaced, and 

 the caterpillars, which were at first simply variable, 

 became polymorphic and then dimorphic, finally 

 returning to monomorphism. 



We thus see the process of transformation still 

 going on, and no doubt can arise as to its 

 inciting causes. When a character can with 

 certainty be ascribed to adaptation, we can explain 

 its origin in no other way than by the action of 



