216 THE EVOLUTION THEOEY 



US appears so to all animals ; yet when we see that all the male birds 

 which are beautifully decorative according to our taste — whether 

 humming-birds, pheasants, birds of Paradise, or rock-cocks {Rupicola 

 crocea)— unfold their ' feather- wheels,' 'fans,' 'collars,' and so forth, 

 before the eyes of the females in the breeding season, and display 

 them in all their brilliance, we must conclude that, in these instances 

 at least, human taste accords with that of the animals. That birds 

 have sharp vision and distinguish colours is well known; it is 

 not for nothing that the 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 Miiller supposed, then even an eye with 13,000 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 flnd 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 



