74 THE EVOLUTION THEOKY 



on which, moreover, the lateral veins are not very conspicuous. 

 Nevertheless the caterpillar enjoys very fair security, as it does not 

 feed through the day, but only in twilight and at night ; it passes the 

 daytime concealed in the dry leaves and earth about the base of the 

 bush. Its resemblance to the leaves is very great, and is increased by 

 the fact that it bears on the last segment a comparatively large 

 orange-coloured spot (r), exactly the colour of the buckthorn berry, 

 which ripens just at the time that the caterpillar attains its full 

 growth. 



But butterflies are as much persecuted, and have as much need 

 of protection, as caterpillars, and among them, too, we find many 

 instances of protective colouring, which are the more interesting in 

 that they occur, as a rule, only on such parts of the body as remain 

 visible when the insect is at rest, which is exactly what we should 

 expect if the coloration has been wrought out in the course of 

 natural selection. But it is well known that the resting position of 

 diurnal Lepidoptera is quite different from that of the nocturnal forms, 

 and is not even the same among all families, and in accordance with 

 this we find the sympathetic colouring occurs on quite different areas 

 in the difierent families. 



The reason why the butterflies only require to be protected by 

 their colour in the sleeping or resting position is that no colour what- 

 ever could make a flying butterfly invisible to its enemies, because 

 the background against which its body shows is continually changing 

 during its flight, and, moreover, the movement alone is enough to 

 betray it, even if it is of a dull colour. 



Thus, in general, only those parts of a butterfly's wing that are 

 invisible at rest could safely bear bright or conspicuous colour, while 

 the visible portions had to acquire sympathetic coloration through 

 natural selection. 



As the diurnal butterflies, when at rest, turn their wings upward 

 and bring them together, it is only the under side which is 

 sympathetically coloured, and that only as far as it is visible, that 

 is, the whole of the posterior wing, and as much of the anterior one 

 as is not covered by it. Many diurnal butterflies, when at rest, fold 

 the anterior wing so far back that only its tip remains visible, and in 

 such cases only this tip is protectively coloured, while in other forms, 

 which have not this habit, almost the whole surface of the wing is 

 sympathetically coloured. 



A very simple protective colouring is exhibited by our 'lemon 

 butterfly' {Rhodocera rhamni), in which the under surface is a 

 whitish yellow, which protects the insect well when it settles on 



