Enough has now been said on correlative changes, to convince us that
they are as a rule to be considered as the expression of some general
internal or physiologic quality, which is not limited to a single organ,
but affects all parts of the organism, provided they are capable of
undergoing the change. Such characters are therefore to be considered as
units, and should be referred to the group of single characters.
Opposed to these are the true compound characters, which consist of
different units. These may be segregated by the production of varieties,
and thereby betray the separate factors of the complex group.
The most beautiful instances of such complex characters are offered by
the colors of some of the most prized garden-flowers. Rarely these are
of a single hue, often two or three shades contribute to the effect, and
in some cases special [149] spots or lines or tracings are to be seen on
a white or on a colored background. That such spots and lines are
separate units is obvious and is demonstrated by the fact that sometimes
spotless varieties occur, which in all other respects have kept the
colors of the species. The complexity of the color is equally evident,
whenever it is built up of constituents of the anthocyan and of the
yellow group. The anthocyan dye is limited to the sap-cavity of the
cells, while the yellow and pure orange colors are fixed in special
organs of the protoplasm.
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