358 TROPICAL NATURE v 
and Mr. Sorby has detected no less than seven distinct 
colouring matters in birds’ eggs, several of which are chem- 
ically related to those of blood and bile. The same colours 
are often produced by quite different substances in different 
groups, as shown by the red of the wing on the burnet-moth 
changing to yellow with muriatic acid, while the red of the 
red-admiral butterfly undergoes no such change. 
These pigmental colours have a different character in 
animals according to their position in the integument. Fol- 
lowing Dr. Hagen’s classification, epidermal colours are those 
which exist in the external chitinised skin of insects, in the 
hairs of mammals, and, partially, in the feathers of birds. 
They are often very deep and rich, and do not fade after 
death. The hypodermal colours are those which are situated 
in the inferior soft layer of the skin. These are often of 
lighter and more vivid tints, and usually fade after death. 
Many of the reds and yellows of butterflies and birds belong 
to this class, as well as the intensely vivid hues of the naked 
skin about the heads of many birds. These pigments some- 
times exude through the pores, forming an evanescent bloom 
on the surface. 
Interference colours are less frequent in the organic world. 
They are caused in two ways: either by reflection from the 
two surfaces of transparent films, as seen in the soap-bubble 
and in thin films of oil on water; or by fine strive which pro- 
duce colours either by reflected or transmitted light, as seen 
in mother-of-pearl and in finely-ruled metallic surfaces. In 
both cases colour is produced by light of one wave-length 
being neutralised, owing to one set of such waves being 
retarded or shifted so as to be half a wave-length behind the 
other set, as may be found explained in any treatise on 
physical optics. The result is, that the complementary colour 
of that neutralised is seen ; and, as the thickness of the film or 
the fineness of the strie undergo slight changes, almost any 
colour can be produced. This is believed to be the origin of 
many of the glossy or metallic tints of insects, as well as 
those of the feathers of some birds. The iridescent colours of 
the wings of dragon-flies are caused by the superposition of 
two or more transparent lamelle; while the shining blue of 
the purple-emperor and other butterflies, and the intensely 
