Yellow flowers in their natural state exhibit but little variation of color. They change most readily to white, and less often to red or blue. Under cultivation sudden variations from yellow to white have been observed. A double yellow hollyhock, according to Darwin, suddenly turned one year into a single white kind, and a chrysanthemum has been seen to bear both yellow and white flowers. It will be observed in the accompanying table that in all families in which yellow flowers are common, white are also common, except in the _Hyperaceæ_, which contain no white-flowered species. Some species of mustard regularly fade to white, while many white flowers show that they are descended from ancestral yellow forms by retaining vestiges of this color on the base of the petals, as in the water-crowfoot. The pale yellow flowers of _Oenothera laciniata_, of the cultivated _Ribes aureum_, and of _Diervilla trifida_ in fading change to rose or red, exhibiting a tendency to develop red coloration. _Aquilegia canadensis_ produces scarlet flowers, which are yellow inside and rarely all over. There are two other species in the Northern flora which exhibit similar coloring, _Lonicera sempervirens_ and _Spigelia marylandica_, and the former is sometimes yellow throughout. _Myosotis_ is at first pale yellow, and changes to sky-blue. But the best illustration of the transition from yellow to blue is exhibited by the violet family; the smallest and simplest species is yellow, the most highly specialized is blue, and all the intermediate stages are presented by _Viola tricolor_. Honey-guides are exceedingly rare among yellow flowers. _Cassia chamæcrista_, which has nearly regular, showy yellow flowers, has two or three petals with a purple spot at base, while four of the anthers are yellow and six purple. It is interesting to compare with this flower the change of color presented by _Arnebia_. When the flower opens, each lobe of the yellow corolla is marked by a dark purple spot, which soon begins to fade, and by the next day has entirely disappeared. _Saxifraga aizoides_ has golden flowers spotted with orange, and attracts a large number of insect visitors, and the yellow violets have their petals marked with dark-brown lines leading to the honey glands. Sulphur-yellow flowers are visited chiefly by bumblebees, and their coloration seems to have been developed by their selective influence from red or purple-flowered ancestors. Müller observed that the sulphur-yellow flowers of _Sempervivum Wulfenii_, which are unlike the primitive yellow of the _Crassulaceæ_, are purple at base. This purple coloring he believed to be a remnant inherited from an earlier purple-flowered form. _Hibiscus trionum_, which is sulphur-yellow with a blackish eye, has perhaps been derived from a red-flowered ancestor, for the three other species of the genus are rose or flesh colored.