In shape and size, as well as in number, the teeth of mammals are very clearly related to the nature of their food in the first place, and secondly to their use as weapons of attack or of defence. When the surface of the cheek-teeth is broad, with low and numerous tubercles, the food of the animal is of a rather soft substance, which yields to a grinding action. Such substances are fruits, nuts, roots, or leaves, which are "triturated" and mixed with the saliva during the process of mastication. Where the vegetable food is coarse grass or tree twigs, requiring long and thorough grinding, transverse ridges of enamel are present on the cheek-teeth, as in elephants, cattle, deer, and rabbits (see Figs. 8, 17, 19). Truly carnivorous animals, which eat the raw carcases of other animals, have a different shape of teeth. Not only do they have large and dagger-like canines or "dog-teeth" as weapons of attack, but the cheek-teeth (very few in number) present a long, sharp-edged ridge running parallel to the length of the jaw, the edges of which in corresponding upper and lower teeth fit and work together like the blades of a pair of scissors. The cats (including the lions, tigers and leopards) have this arrangement in perfection (see Figs. 21 and 22). They cut the bones and muscles of their prey into great lumps with the scissor-like cheek-teeth, and swallow great pieces whole without mastication. Insect-eating mammals have cheek-teeth with three or four sharp-pointed tubercles standing up on the surface. They break the hard-shelled insects and swallow them rapidly. The fish-eating whales have an immense number of peg-like pointed teeth only. These serve as do those of the seals--merely to catch and grip the fish, which are swallowed whole. It is quite clear that man's cheek-teeth do not enable him to cut lumps of meat and bone from raw carcases and swallow them whole, nor to grip live fish and swallow them straight off (Pl. VI). They are broad, square-surfaced teeth, with four or fewer low rounded tubercles fitted to crush soft food, as are those of monkeys (see Pl. VII and its description). And there can be no doubt that man fed originally, like monkeys, on easily crushed fruits, nuts, and roots. He could not eat like a cat.