There is one genus of diurnal butterflies which seems to contradict the law that all the surface that is visible in the resting position exhibits the protective coloration--the South American wood-butterflies of the genus _Ageronia_. They have on the upper surface a very complicated bark-like pattern of mingled grey on grey, and this confirms the usual rule, for we know that these butterflies--a striking exception among all the other diurnal forms--settle with outspread wings on the trunk of a tree in exactly the same attitude as many of the nocturnal Lepidoptera of the family of the Loopers or Geometridæ, in which the upper surface is also deceptively like the bark of the tree on which they rest. [Illustration: FIG. 9. _Hebomoja glaucippe_, from India; under surface. _A_, in flight. _B_, in resting attitude.] In all the nocturnal Lepidoptera it is the _upper_ side of the wing which is sympathetically coloured, if protective coloration has been developed at all. In all the Sphingidæ, many 'Owls' and Bombycidæ, the anterior wings are grey banded with darker zigzag lines, and mottled with many shades of black, grey, yellow, red, and even violet. As the anterior wings cover the body and the posterior wings like a roof, they make the resting insect very inconspicuous when it has settled on wooden fences, trunks of trees, or even old timber. When bright colours--red, yellow, or blue--occur in these moths it is always on the posterior wings, which are covered when at rest. This can best be observed in the species of the genus _Catocala_. Let us now, however, interrupt our survey of the facts for a moment, and let us inquire whether all the cases of protective colouring in Lepidoptera we have considered can be referred to natural selection, or whether it is not conceivable that other causes may have evoked them. [Illustration: FIG. 10. _Xylina vetusta_, after Rösel. _A_, in flight. _B_, at rest.]