service berries and many other berries suitable for birds are red, the mistletoe berries white, in contrast to the evergreen foliage of this plant, the juniper berries black so that they stand out amid the snows of winter; in this direction, then, there is no difficulty in the way of sexual selection. Even among much lower animals, like the butterflies, there seems to me no reason for the assumption that they do not see the gorgeous colours and often very complicated markings, the bars and eye-spots, on the wings of their fellows of the same species. Of course if each facet of the insect eye contributed only a single visual impression, as Johannes Müller supposed, then even an eye with 12,000 facets would give but a rough and ill-defined picture of objects more than a few feet away, and I confess that for a long time I regarded this as an obstacle in the way of referring the sexual dimorphism of butterflies to processes of selection. But we now know, through Exner, that this is not the case; we know that each facet gives a little picture, and not an 'inverted' but an 'upright' one, and experiment with the excised insect eye has directly shown that it throws on a photographic plate a tolerably clear image of even distant objects, such as the frame of a window, a large letter painted on the window, or even a church tower visible through it. Furthermore, the structure of the eye allows of incomparably clearer vision of near objects, for in that case the eyes act like lenses, and reveal much more minute details than we ourselves are able to make out. Here again, therefore, there is no obstacle to the Darwinian hypothesis of a choice on the part of the females, for although it cannot be demonstrated from the structure of the eye itself that insects see colour, and that colours have a specially exciting influence on them, yet we can deduce this with certainty from the phenomena of their life. The butterflies fly to gaily coloured flowers, and as they find in them their food, the nectar of the flowers, we may take for granted that the sight of the colour of their food-providing plants is associated with an agreeable sensation, and this is an indication that similar colours in their fellows may awaken similar agreeable sensations. [Illustration: FIG. 53. Scent-scales of diurnal butterflies. _a_, of Pieris. _b_, of Argynnis paphia. _c_, of a Satyrid. _d_, of Lycæna. All highly magnified.]