LAW OF PROGRESSIVE COLOURATION. 51 



inquire into the phenomena of relapse and retrogres- 

 sion. Nevertheless, even at the present stage of our 

 inquiry, the composites afford some excellent evidence. 

 In certain ways they may be regarded as the very 

 highest race of flowering plants. Not only are their 

 petals united into a tubular corolla, but their blossoms 

 are compounded into large bunches or groups of a 

 very showy and attractive sort. Each flower-head 

 here consists of a number of small florets, crowded 

 together so as to resemble a single blossom. So far 

 as our present purpose is concerned, they fall natur- 

 ally into three groups — Jussieu's old-fashioned sub- 

 orders of Ligidatcs, CynaroidecB, and Cotyinbiferce, 

 which are quite sufficient for all ordinary objects of 

 botanical study. The first, or ligulate tribe, is that 

 of the dandelions or hawkweeds, with open florets, 

 fertilised, as a rule, by very small insects ; and these 

 are generally yellow, with only a very few divergent 

 species. They will receive further notice hereafter. 

 The second, or cynaroid tribe, is that of the thistle- 

 heads, visited by an immense number of insects, in- 

 cluding the bees ; and these are almost all purple, 

 while some highly-evolved species, like the corn-flower 

 or blue-bottle {Centaurea cyamis, Fig. 17) and the true 

 artichoke {Cynara scolynius), are bright blue. The 

 third, or corymbiferous tribe, is that of the daisies and 

 asters, with tubular central florets and long flattened 

 outer rays ; and these demand a closer examination 

 here. 



The central florets of the daisy tribe, as a rule, are 

 bright golden ; a fact which shows pretty certainly 

 that they are descended from a common ancestor 

 who was also yellow. Moreover, these yellow florets 



E 2 



