LA W OF PROGRESSIVE COLOURATION. 23 



have undergone most modification have their colours 

 most profoundly altered. In this way, we may put 

 it down as a general rule (to be tested hereafter) 

 that the least developed flowers are usually yellow 

 or white ; those which have undergone a little 

 more modification are usually pink or red ; and 

 those which have been most highly specialised of 

 any are usually purple, lilac, or blue. Absolute 

 deep ultramarine probably marks the highest level 

 of all. 



On the other hand, Mr. Wallace's principle also 

 explains why the bees and butterflies should prefer 

 these specialised colours to all others, and should 

 therefore select the flowers which display them by 

 preference over any less developed types. For bees 

 and butterflies are the most highly adapted of all 

 insects to honey-seeking and flower-feeding. They 

 have themselves -.on their side undergone the largest 

 amount of specialisation for that particular function. 

 And if the more specialised and modified flowers, 

 which gradually fitted their forms and the position of 

 their honey-glands to the forms of the bees or 

 butterflies, showed a natural tendency to pass from 

 yellow through pink and red to purple and blue, it 

 would follow that the insects which were being evolved 

 side by side with them, and which were aiding at the 

 same time in their evolution, would grow to recognise 

 these developed colours as the visible symbols of 

 those flowers from which they could obtain the largest 

 amount of honey with the least possible trouble. 

 Thus it would finally result that the ordinary un- 

 specialised flowers, which depended upon small insect 

 riff-raff, would be mostly left yellow or white ; those 



