primrose, _Oenothera tetraptera_, has white
flowers in its first stage, and red ones at a later period of
development. _Cobea scandens_ goes from white to violet; _Hibiscus
mutabilis_ from white through flesh-colored to red. The common Virginia
stock of our gardens _(Malcolmia)_ often opens of a pale yellowish
green, then becomes faintly pink; afterward deepens into bright red; and
fades away at the last into mauve or blue. Fritz Mueller's _Lantana_ is
yellow on its first day, orange on its second, and purple on the third.
The whole family of _Boraginaceae_ begin by being pink and end with
being blue. The garden convolvulus opens a blushing white and passes
into full purple. In all these and many other cases the general
direction of the changes is the same. They are usually set down as due
to varying degrees of oxidation in the pigmentary matter. If this be so,
there is a good reason why bees should be specially fond of blue, and
why blue flowers should be specially adapted for fertilization by their
aid. For Mr. A.R. Wallace has shown that color is most apt to appear or
to vary in those parts of plants or animals which have undergone the
highest amount of modification. The markings of the peacock and the
argus pheasant come out upon their immensely developed secondary
tail-feathers or wing-plumes; the metallic hues of sun-birds, or
humming-birds, show themselves upon their highly specialized crests,
gorgets, or lappets. It is the same with the hackles of fowls, the head
ornaments of fruit-pigeons, and the bills of toucans. The most exquisite
colors in the insect world are those which are developed on the greatly
expanded and delicately feathered wings of butterflies; and the
eye-spots which adorn a few species are usually found on their very
highly modified swallow-tail appendages. So too with flowers: those
which have undergone most modification have their colors most profoundly
altered. In this way, we may put it down as a general rule (to be tested
hereafter) that the least developed flowers are usually yellow or white;
those which have undergone a little more modification are usually pink
or red; and those which have been most highly specialized of any are
usually purple, lilac, or blue. Absolute deep ultramarine probably marks
the highest level of all.

On the other hand, Mr. Wallace's principle also explains why the bees
and butterflies should prefer these specialized colors to all others,
and should therefore s