182 THE STRUCTURE OF FLOWERS. 



shades. Similarly the orange or buff-yellow Rhododendron 

 Javanicum has been split up into various reds ; the white 

 having, so to say, eliminated the yellow. 



The subsequent effect of crossing with regard to flowers 

 is variety. With this fact florists and horticulturists are 

 familiar : for as soon as crossed or hybrid seedlings are 

 raised the varieties of colouring become infinite. It has 

 been observed that the " spots " are more persistent than 

 the base-colour of the flower. This fact agrees with the 

 theory advanced that they have, whenever they occur as 

 guides or path-finders, been determined by the insects and 

 then become hereditary as much as the shape of the flower 

 itself ; and as that is maintained much more persistently 

 than general colouring, so is that specialized colouring which 

 has been equally due to insect agency. 



With regard to the correlations which exist between 

 colours and insect visitors, Miiller especially has observed 

 several. Thus beetles seem to affect yellows, e.g. Thalictrum 

 and Galium verum ; wasps and carrion insects, reddish- 

 browns, such as of Gomarum, Ejoipactis, etc., while the more 

 intelligent bees, etc., delight in purples and blues ; and it is 

 thought that their selective agency has determined the sur- 

 vival of such special colours as they prefer. This has been 

 probably the case, but we still want to know what is the 

 immediate cause which induces one colour to change to 

 another. 



As high colouring or conspicuousness if the flower be 

 white is due to insects, so pale colouring and inconspicuous- 

 ness is due to their absence ; but what the nature of the 

 stimulus is we cannot tell. It enhances the assimilative 

 powers ; for the crossed plants, as Mr. Darwin abundantly 

 proved, are usually larger plants. It usually infuses some 

 of the characters, floral or foliar, of the male parent — bat 



