238 Conmposite—Dahlia: 
offers one of the most striking instances of the variability o7 
species under domestication, which is exemplified not only in 
the modification of the disk-florets but also in the wide range of 
colours. But so little is understood of the real nature of vege- 
table colouring matter and the cause of its variability, and to 
what influences the changes must be ascribed, that we cannot 
Fig. 127. Dahlia coccinea, (3 nat. size.) 
correctly estimate the importance of this phenomenon. We 
know that pure white flowers exist, and that various shades and 
tints of yellow, scarlet, and purple, and combinations of these 
colours, are common; but we are not sure whether these colours 
are not also found in natural varieties. There is evidently a 
limit in the production of colours, as nothing approaching blue 
