Colour vision is not an all-or-none state. Many groups of animals can distinguish colour, but in different ways. In mammals, for instance, some groups are without colour vision, and some are dichromats: they have two kinds of cones, and cannot see ultraviolet, red and orange light. It is thought that the mammals which survived though the Jurassic period were small nocturnal burrowing animals with little need to see colour. Later, after the extinction of the dinosaurs, many lines adapted to be more active in the daytime. Most developed a form of colour vision, which is much more useful in the daytime. Primates developed full (trichromacy) colour vision. For them, the ability to distinguish the colour of fruit and leaves is almost essential.