The Crocodile is colourless, or at least is monochromatic; not so many of the Lizards, which rival the birds in the vividness of their hues. With the birds the colours undergo no changes save such as are due to the incidence of light; with the Lizards, however, the bare skin is exposed and this can, as it were, be made to blush with all the colours of the rainbow. Having regard to what has been said already as to the sources of this coloration it is not surprising to note that here also the males are the more vividly coloured whenever the sexes differ in this particular. And further, it is among the most vividly coloured males that most animated displays take place when the endeavour is being made to excite the amorous instincts of the females. The males of the genus Sitana are very brightly garnished. They possess a large throat pouch, coloured blue, black, and red when expanded, and this occurs only during moments of excitement, whether this is due to fear or pleasure. And at the same time the vividness of the coloration is greatly increased. No such secondary sexual characters are present in the female. A variant on the throat pouch, of a much more striking character, is displayed by the Frilled Lizard (_Chlamydosaurus kingi_), wherein the tongue bones have become enormously elongated so as to project backwards on each side of the body almost as far as the base of the tail. With them they have carried a thin fold of skin, so that whenever the mouth is opened these bones stand out at right angles to the head and display a circular fold of skin stretched as it were on rods; or they may be compared to the ribs of an umbrella. The great Elizabethan frill thus formed, is displayed only during moments of great excitement, and the open mouth, at such times, is flushed with a vivid red, which, contrasting with the teeth, gives a very terrifying aspect to prospective enemies, and doubtless also proves a valuable asset as a “secondary sexual character.”