E ELAPSE AND RETROGRESSION. 77 



at the bases of their petals. The reader is recom- 

 mended to notice this point for himself in the 

 commoner white or pink polypetalous flowers. 



With the light thus cast upon the question to guide 

 us, we may pass on to the general consideration of 

 Retrogression in colours. Certain species of advanced 

 families have apparently found it advantageous in 

 certain circumstances regularly and consistently to 

 revert to colours lower in the scale than the normal hue 

 of their congeners. The reasons for such Retrogression 

 are often easy enough to understand. 



We may take the evening campion {LycJmis vesper- 

 tind) as a good example (Fig. 15). This white flower, 

 as we saw, is evidently descended from the red day 

 campion [LycJinis diuriia), because it is still often pale 

 pink, especially towards the centre, verging into white 

 at the edge. But it has found it convenient to attract 

 moths and be icrtiliscd by them ; and so it has lost its 

 pinkness, because white is naturally the colour best 

 seen by crepuscular insects in the dusky light of 

 evening. It is scented at nightfall, and readily allures 

 many moths by the combined attraction of sight and 

 smell. Sir John Lubbock notes that such evening flowers 

 never have any spots or lines as honey-guides on the 

 petals, because such marks could not be seen at night, 

 and would therefore be useless. All the other British 

 species oi Lychnis are red, pink, or purple. 



The evening primrose {Oenothera bicnuis), now 

 naturalised in England, is another excellent instance 

 of the same sort. It belongs to the family of the 

 Ofiagraccce, which are highly evolved polypetalous 

 plants, with the petals reduced to four or two in 

 number, and placed above instead of below the ovary. 



