LA W OF PROGRESSIVE COLOURA TION. 51 



exhibits very beautifully the importance as regards 

 colouration of mere expansion in the petals. Taking 

 them as a whole we may say that the smallest petals 

 in the rose family are generally yellow; the next in 

 size are generally white ; the third in order are gene- 

 rally pink ; and the largest are generally rose-coloured 

 or crimson. 



At the same time, the roses as a whole, being a 

 relatively simple family, with regular symmetrical 

 flowers of the separate or polypetalous type, have 

 never risen to the stage of producing blue petals. 

 That, probably, is why our florists cannot turn out a 

 blue rose. It is easy enough to make roses or any 

 other blossoms vary within their own natural limits, 

 revert to any earlier form or colour through which 

 they have previously passed ; but it is difficult or 

 impossible to make them take a step which they have 

 never yet naturally taken. Hence florists generally 

 find the most developed flowers are also the most 

 variable and plastic in colour ; and hence, too, we can 

 get red, pink, white, straw-coloured, or yellow roses, 

 but not blue ones. This would seem to be the 

 historical truth underlying De Candolle's division of 

 flowers into a xanthic and cyanic series. Of course, 

 there is nothing to prevent florists from developing a 

 blue rose in the same way as the insects would do it, 

 by gradually selecting and preserving the most bluish 

 or slate-coloured among their pink or crimson kinds. 

 But it would appear from the comparative rarity of 

 blue flowers in nature that the spontaneous variations 

 which make towards blue are far less frequent than 

 those which make towards pink, red, purple, or 

 orange. 



