LA W OF PROGRESSIVE COLO URA TION. 4 r 



Our own beautiful purple English corn-cockle 

 (Z. githagd), is a highly developed campion, so 

 specialised that only butterflies can reach its honey 

 with their long tongues, as the nectaries are situated 

 at the bottom of the tube. Two other species of 

 campion, however, show us interestingly the way 

 in which variations of colour may occur in a retro- 

 grade direction even among highly evolved forms. 



Fig. 15.— Flower of night campion (^Lychnis vespertina) ; white. 



One of them, the day lychnis, {L. diiirna), has red, 

 scentless flowers, opening in the morning, and it is 

 chiefly fertilized by diurnal butterflies. But its de- 

 scendant, the night lychnis (Z. vespertina), has taken 

 to fertilization by means of moths ; and as moths can 

 only see white flowers it has become white (Fig. 1 5), and 

 has acquired a faint perfume as an extra attraction. 

 Still, the change has not yet become fully organised 



