IS7S.J 
Colouration of the Organic World. 
35 
It is well known that what we designate as colour may 
be produced either by the interference or by the absorption 
of rays of light, and hence the colours of animals may be 
divided into two well-marked classes. On the one hand, 
especially in birds and inserts, we find hues which are iri- 
descent changeable according to the relative positions of the 
observer and of the light, and are possessed of an intense, 
so-called, metallic lustre. Such colours — to take familiar 
examples — may be seen in the plumage of the peacock, of 
the starling, on the wings of the “ purple emperor” butter- 
fly ( Apatura Iris), on the entire coating of the rose-beetle 
(Cetonia aurata), of the fire-wasp ( Chryseis ignita), and of 
many other common native inserts. In the vegetable king- 
dom they may be pronounced unknown. Such colours are 
due to the interference of certain rays of light, whether re- 
flected from superimposed transparent films or reflected from 
or refraCted through minute striae. These colours are per- 
manent, even on the most prolonged exposure to air, to 
atmospheric moisture, or to full sunlight. Unless the very 
texture of the feather, the wing-scale, the elytron, &c., be 
destroyed by putrefaction or combustion, the colour remains 
unhurt. Nor can we by any means extract from such 
coloured surfaces a dye or pigment capable of being applied 
to other objects. 
On the other hand, there are colours which do not change 
their shade from whatever position they are regarded, and 
which possess little of that intense lustre which marks the 
former class. To this kind belong the colours of all flowers, 
of caterpillars, of the great majority of our native butterflies 
and moths, and, in short, of the vast bulk of organic beings. 
These colours are due to the absorption of certain of the 
rays of light, such absorption being effected by substances 
known as pigments, and capable, when present in sufficient 
quantity, of being extracted by solvents, and used to dye or 
stain other bodies. Such colours have not the permanence 
of the first-mentioned class. Every entomologist knows 
that if a case of butterflies be kept constantly exposed to 
the sun, or even to diffused daylight, then — no matter how 
completely air, damp, and mites may be excluded — the spe- 
cimens fade, even though the minute scales which clothe 
the wings may still be found in their places. Yet the golden 
spots on the wings of the Plusice and the pearl-mother 
markings of the “ fritillaries ” remain unchanged. The 
colours of most other inseCts behave in a very similar 
manner. Beetles are generally supposed to wear a more 
permanent livery; but every Coleopterist must have observed 
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