80 THE EVOLUTION THEORY 



that the covered parts take on any indifferent colour, which might 

 be readily afforded by the metabolism of the insect. But this much 

 is certain, that the covered parts would be green, if that were 

 advantageous to the survival of the species, just as the under surface 

 of the wings of some diurnal butterflies is green. Had it been 

 required, the green colour would have resulted in the course of 

 natural selection, just as it has resulted in the most different parts 

 of the most diverse insects, even in those whose development takes 

 place entirely removed from the influence of light. Therein lies the 

 difference between our interpretation and that of Brunner von 

 Wattenwyl : without natural selection no explanation of this case 

 is possible. 



Hitherto I have spoken only of the diurnal butterflies in which 

 the anterior wings show an extension of the protective colouring 

 which marks the whole surface of the posterior wings, and it was 

 always the tips of the anterior wings that were thus coloured. But 



3 



Fig. 12. Notodonta camelina, after Eosel. A, in flight. B, at rest. 



among the nocturnal Lepidoptera there are corresponding cases, in 

 which a little tip of the posterior wing forms the continuation of 

 the protective surface of the anterior wing. Some species of 

 Notodonta and allied genera show in the posterior corner of the 

 otherwise whitish posterior wings a little grey spot, and a hair tuft 

 which in colour, and — when it is big enough — in marking, exactly 

 resembles the protectively coloured anterior wings (Fig. la). The 

 ' why ' is at once clear, when one looks at the insect in the resting 

 position, for only this little corner of the wing projects beyond the 

 covering anterior wing. This has been regarded as telling against 

 natural selection, for such a little spot could not possibly, by its 

 colour, turn the scale as to the life or death of the individual, and 

 so could not be selected. But one might say the same of the 

 tip of the anterior wing in the diurnal forms, although there 

 the protective surface is larger, often much larger. But who is to 

 decide how large an exposed, unprotected spot must be in order to 

 attract the attention of an enemy on the look-out for food? Or 



