SEXUAL SELECTION 237 



lines of every shape and colour. On the other hand, the upper surface 

 of the anterior wings in the hawk-moths and the Noctuidse exhibits 

 protective coloration, and is made up of curious zigzag complex lines, 

 strokes, and spots, so that it resembles the bark of a tree or a bit of an 

 old wooden fence— a painting, like the modern impressionist work, 

 which, with an apparently meaningless confusion of colour splashes, 

 conveys a perfect impression even of the details of a landscape. In the 

 owl-moths (Noctuidse) the wing surfaces, which are brightly coloured, 

 are simple, almost crude, in pattern, as in the moths of the genus 

 Gatocala, with their red, blue, or yellow posterior wings, traversed 

 by a large black band ; while in the Geometer-moths, whose wings 

 are spread out flat when at rest, the protective upper surface of 

 all four wings is covered with a complex pattern of lines, spots, and 

 streaks in diS'erent shades of grey, yellow, white, and black, so that it 

 bears a deceptive resemblance to the bark of a tree or the side of a 

 wall. For a long time I could not understand how such a definite 

 and constant pattern could arise through natural selection if it was 

 a case of mimicking the impression of bark or of any other irregularly 

 covered surface, the colours of which are not mingled in exactly the 

 same way everywhere. But now I think I understand it; for in the 

 apparently meaningless colour - splashes of an 'impressionist' land- 

 scape the different splashes must be exactly where they are, otherwise 

 on stepping back from the picture one would see, not a Haarlem 

 hyacinth-field, or an avenue of poplars with their golden autumn 

 leaves, but a mere unintelligible daub. It is the type of the colour- 

 pattern that must be attained, and in nature this is attained very 

 slowly, step by step, spot after spot, and therefore, obviously, no 

 correct stroke once attained will be given up again, since, in combi- 

 nation with the rest, it secures the proper type of colour-pattern. 

 Only thus, it seems to me, can we understand how apparently 

 meaningless lines, like the figures 1840 on the under surface of 

 Vanessa atalarda, could have become a constant characteristic of 

 the species. 



To sum up briefly, we may say that sexual selection is a much 

 more powerful factor in transformation than we should at first be 

 inclined to believe. It cannot, of course, have been operative in the 

 case of plants, nor can it be taken into consideration in regard to the 

 lower animals, for these, like the plants, do not pair, or, at any rate, 

 do so without any possibility of choice. Animals which live on the 

 sea-floor, or which are attached there, must simply liberate their 

 reproductive cells into the water, and cannot secure that they unite 

 with those of this or that individual. This is the case among sponges. 



