38 THE COLOURS OF ANIMALS 



places, where the obvious thickness would destroy the 

 resemblance to a leaf, the whole effect of the round- 

 ness is neutralised by increased lightness, so disposed 

 as just to compensate for the shadow by which alone 

 we judge of the roundness of small objects. The 

 degree of whiteness is produced by the relative abun- 

 dance of white dots and a fine white marbling of the 

 surface, which is everywhere present mingled with the 

 green. The effect is, in fact, produced by a process 

 exactly analogous to stippling. The degree of lightness 

 produced in this way exactly corresponds to the angle 

 of the slope, which, of course, determines the depth 

 of the shadow. By this beautiful and simple method 

 the pupa appears to be as flat as a leaf which is only 

 a small fraction of 1 mm. in thickness. 



Although the effect which I have just described 

 could not have been surpassed by the efforts of an 

 artist, it is precisely the result which can be most 

 readily explained by the unaided operation of natural 

 selection. The minute white markings are present 

 over the whole surface, and their number and size 

 must be subject to continual variation ; in fact, it is 

 quite certain that no two individuals are alike in these 

 respects. The increased protection afforded by their 

 more appropriate distribution in certain individuals 

 would clearly lead to the survival of the latter, while 

 the same process continued in each generation would 

 lead to the elaborate ^nd beautiful form of adaptation 

 which is now witnessed in this species. 



