The question next arises, as to whether the causes originating the phenomena are not the same as those to which we ascribe the change of winter and summer covering in so many mammalia and birds--whether the change of colour and marking does not depend, in this as in the other cases, upon the _indirect action_ of external conditions of life, i.e., on adaptation through natural selection. We are certainly correct in ascribing white coloration to adaptation[7]--as with the ptarmigan, which is white in winter and of a grey-brown in summer, both colours of the species being evidently of important use. It might be imagined that analogous phenomena occur in butterflies, with the difference that the change of colour, instead of taking place in the same brood, alternates in different broods.[8] The nature of the difference which occurs in seasonal dimorphism, however, decidedly excludes this view; and moreover, the environment of butterflies presents such similar features, whether they emerge in spring or in summer, that all notions that we may be dealing with adaptational colours must be entirely abandoned. I have elsewhere[9] endeavoured to show that butterflies in general are not coloured protectively during flight, for the double reason that the colour of the background to which they are exposed continually changes, and because, even with the best adaptation to the background, the fluttering motion of the wings would betray them to the eyes of their enemies.[10] I attempted also to prove at the same time that the diurnal Lepidoptera of our temperate zone have few enemies which pursue them when on the wing, but that they are subject to many attacks during their period of repose. In support of this last statement I may here adduce an instance. In the summer of 1869 I placed about seventy specimens of _Araschnia Prorsa_ in a spacious case, plentifully supplied with flowers. Although the insects found themselves quite at home, and settled about the flowers in very fine weather (one pair copulated, and the female laid eggs), yet I found some dead and mangled every morning. This decimation continued--many disappearing entirely without my being able to find their remains--until after the ninth day, when they had all, with one exception, been slain by their nocturnal foes--probably spiders and _Opilionidæ_. Diurnal Lepidoptera in a position of rest are especially exposed to hostile attacks. In this position, as is well known, their wings are closed upright, and it is evident that the adaptational colours on the under side are displayed, as is most clearly shown by many of our native species.[11]