Colours of Animals

COLOURS OF ANIMALS. Pigments or colour-producing substances have many roles in the life of organisms. Some are of fundamental physiological importance, like the greenish pig ments (chlorophylls) of most plants and the reddish pigment (haemoglobin) of the blood in backboned animals. But it is necessary to draw a distinction between pigmentation and colora tion. Thus it is useful to the ermine (Mustela ermines) and to the mountain hare (Lepus variabilis) to have a white colour in winter, but there is no pigment involved, the whiteness being due to gas-vacuoles in the hairs. Conversely, some very important pigments, such as cytochrome (see below), are practically with out colour.

Ordinary visible light consists of

rays of different wave-lengths, each of which produces in our eye and brain a particular colour sensation, such as red or green. The shortest are the violet rays, and the longest, at the other end of the spectrum, are the red rays. If the colours, which are the vital results of the rays of different wave-lengths, are written down in this way : Violet Green Indigo Yellow Blue Orange Green Red the pairs placed opposite each other are called "complementary colours" ("green" appearing twice because there are several "greens"), and their relation to one another is important in under standing colouring. If by some process of filtering we can subtract from the complete white light any one member, e.g., red, then the light will appear green—the colour complementary to that filtered out. The fact to be emphasized is that a pigment or colour producing substance is one which interferes with the whiteness of ordinary light. The molecule of the pigment is able to absorb or to reflect only certain parts of the visible spectrum. On the other hand the interference with the white light may be wholly due to the physical structure of the substance, e.g., when it presents a finely grated or finely laminated surface.

This leads to an important distinction in animals between (a) structural or physical coloration, (b) pigmentary or chemical coloration, and (c) a combination of the two. (a) Structural or physical coloration, due to the texture or architecture of the sur face of the animal, may be illustrated by the mother-of-pearl (nacre) in the interior of a mollusc's shell, where the lime and an organic matrix are laid down (conchin) in very fine laminae. If the shell is pounded, the iridescence disappears. (b) Chemical or pigmentary coloration, due to the presence of a substance whose molecules absorb or reflect only a certain part of the visible spectrum, may be illustrated by the reddish pigment of our blood, the blackish pigment of dark fur or feathers, and the yellowish pigment of yolk of egg. (c) But the finest coloration in animals is due to a combination of physical and pigmentary colour, as in the blue feathers of some parrots, the metallic green wing-covers of some beetles, the blue scales of some butterflies, and the iridescent bristles of Aphrodite, the sea-mouse. When physical structure is an accessory factor, in addition to chemical pigmenta tion, the colour of the surface changes as it is moved about, as is familiar in the "eye" of the peacock's feather. When physical structure enhances the effect of a pigment, it may result in a colour, notably blue or green, quite different from the colour, often brownish, of the pigmented substance itself. Thus a butter fly may be burnished blue above and pale brown below, yet with the same pigment on both surfaces.