• 1
    The paper deals with the morphology and evolution of feathers of structural colours based on Tyndall's principle (short wavelengths of light are scattered from a cloudy medium, underlaid by pigmentations that absorb long wavelengths).
  • 2
    The texture of the cloudy medium, usually of medullary origin, represents a dimensional extreme of vacuolization in feathers without Tyndall colour.
  • 3
    Concealed bases of Tyndall-coloured feathers show primitive conditions, as found in dull-coloured feathers. Transitions between bases and bright terminal regions are interpreted as evolutionary steps, ontogenetically repeated in retrogression.
  • 4
    There are few alternative configurations in pigmentation (1) of medullary or cortical portions, each of them evolved convergently in heterogeneous families: convergence is evident from differences in baso-terminal transitions (3).
  • 5
    Divergence of related species (or of plumage regions in the same species) in late evolutionary phases is suggested by agreement in transitional, but differences in terminal, configurations.
  • 6
    Evolution of alternative configurations may be controlled by available quantities of cells: these are sometimes correlated with the body volume.
  • 7
    The taxonomic significance of Tyndall configurations has been discussed.
  • 8
    Convergent evolution (4) of configurations in Alcedinidae and Thraupidae has been studied in detail.
  • 9
    Double medullae in Alcedinidae and Thraupidae may represent stabilization of fluctuant conditions in other families.